In order to relieve the contradiction of dynamic performance between different loading conditions, a two-stage pressure hydro-pneumatic spring which is designed for off-load vehicle is proposed. Taking the compressibility of oil into consideration, a nonlinear mathematical model is established. A test bench for hydro-pneumatic spring is built and the accuracy of the model is verified. The one-quarter model of vehicle is built up to compare the simulations of vehicles equipped with single-accumulator hydro-pneumatic suspension to that with two-stage pressure hydro-pneumatic suspension under random road excitation. The results show that, compared with the former situation, the body acceleration root mean square (RMS) value under the latter condition decreases by 20.1% (paved road/full-load) and 10.7% (off-road/empty-load) respectively, and the tire dynamic load decreases by 36.8% and 10.4% respectively. Although the RMS value of dynamic travel of suspension increases by 11.8% and 1.9%, the probability of collision to set blocks is less than 0.1%, remaining in a reasonable range.